1. Technical Field
The present invention relates to an image forming apparatus which uses an electrophotographic method to form an image on paper and output it, like copying machines and printers.
2. Description of the Prior Art
An image forming apparatus using the electrophotographic method generally forms an image on paper by forming an electrostatic latent image corresponding to an image signal on a photosensitive body, developing the latent image into a toner image, transferring the toner image onto the paper, and fusing the toner image on the paper.
In such an image forming apparatus, sheets of paper stacked on a paper tray are usually picked up one by one and transported toward a toner image transfer position. During the paper transporting process, any difference between the timing at which the paper reaches the image transfer position and the timing at which the toner image reaches there will result in a shift of the toner image position on the paper from the desired position. When an image is formed over an entire surface of the paper, for example, edge dimensions become uneven, thus significantly degrading the appearance of the formed image.
For this reason, some image forming apparatuses incorporate into the paper transporting system a mechanism for adjusting the timing of the paper reaching the image transfer position. For example, there is a construction in which, as shown in FIG. 19, two paper transporting rollers A, B are arranged along the paper transporting direction of the paper P. In this construction, the paper P fed by the paper transporting roller B on the upstream side is struck against the paper transporting roller A on the downstream side which is at rest and then the timing at which to start rotating the paper transporting roller A is adjusted to match the timing at which the paper arrives at the transfer position to the toner image arrival timing.
In the conventional image forming apparatus described above, however, as shown in FIGS. 20A and 20B the way the paper engages the paper transporting roller A at rest may vary depending on the curled state of the lead edge of the paper, causing the paper position relative to the paper transporting roller A to change from one sheet of paper to another, as shown in FIGS. 21A and 21B. This may result in a difference in the transfer position arrival timing between the paper and the toner image.
To deal with this problem, a so-called nonstop servo registration control may be adopted which temporarily stops the paper in front of the paper transporting roller A and rotates the paper transporting roller A as the lead edge of the paper reaches the paper transporting roller A. This will eliminate possible variations in the paper engagement condition and thereby improves the precision of the paper position relative to the paper transporting roller A.
In this case, while the precision of the timing of the paper arriving at the transfer position improves, variations in the toner image arrival timing become a serious problem. That is, when variations in the toner image arrival timing occur due to the manufacturing precision and assembly errors of components of the image forming apparatus or their changes with elapse of time, it becomes difficult to match the paper arrival timing with the toner image arrival timing, even with an improved precision of the timing of the paper arriving at the transfer position. Especially in an image forming apparatus that uses an image carrying belt, such as a photosensitive belt and an intermediate transfer belt, variations in the toner image arrival timing increase because the image carrying belt elongates or contracts in response to changes in temperature, humidity or belt tension.
Therefore, as shown in FIG. 22A or 22B, when the nonstop servo registration control is adopted, the amount of variation in the toner image arrival timing needs to be determined by detecting the position of the front end of the toner image on an image carrier 41, such as a photosensitive drum or an intermediate transfer belt, by an image front end reading sensor 42 provided near the toner image transfer position. That is, for the image front end reading sensor 42 to detect the front end position of the toner image, it is necessary to form a toner patch on the image carrier 41 for reading the timing, or form a predetermined marking (such as a notch) on the image carrier 41.
The formation of the toner patch on the image carrier 41 in turn requires a pattern generator, control, cleaning, and others, leading to an increase in the apparatus cost and in the development man-hours. Further, if the image front end reading sensor 42 is of a general type, because its reading sensitivity characteristic depends on the color of the toner patch or marking, a satisfactory reading sensitivity characteristic may not be obtained when a single color image (toner patch) is formed. Another problem is that when a marking is formed on the image carrier 41 in advance, the pitch of the marking cannot be changed, thus greatly lowering the productivity of image forming.
The image front end reading sensor 42 is arranged close to the image carrier 41 in order to be able to read the toner patch or marking on the image carrier 41. This renders the toner on the image carrier 41 more likely to be scattered and the scattered toner may contaminate the marking, resulting in a degraded detection precision or even a failure to detect.